The present invention is concerned with the formulation and application of additives to cold or thermosetting resin compositions containing a curable resin such as urea-formaldehyde, melamine formaldehyde, phenol formaldehyde or any condensation product containing any mixture or combination, copolymer or blend thereof. The purpose is to retain and exploit all mechanical, chemical and economic advantages of these resins, especially those formulations containing a significant molar excess of formaldehyde, while reducing the release and odor of formaldehyde of such products during the manufacture thereof and in the finished state.
There is an increasing demand for strong and economical adhesives and other resin for use in making bonded wood and other products such as insulating foams. In such products, cured resins substantially determine the nature and quality of the resulting product. New building technology and environmental concern increase demands that such materials reliably fulfill a variety of exacting standards.
Ideally, ingredients for such resins should be readily available, and should be mixed and formulated easily, preferably using customary procedure or simple modifications thereof. For this purpose the resin should have a long storage life, should be quick setting on application and should be non-odorous and non-toxic and useable with standard equipment. Additionally, the bonded product obtained with the adhesive should have good strength, elasticity, durability and acceptable color. Furthermore, the resin should be useful over a wide range of working conditions, because, even though good bond strength may be possible under certain conditions, the resin may not be acceptable if such results cannot be readily achieved over a wide variation of operations so as to accommodate accepted methods used for specific products, or as necessary to meet local production standards and methods, or the use of different types of materials. Finally, the adhesive should fulfill environmental quality considerations, the highest standards of industrial hygiene, and its application should not be costly.
The art of formulating and applying urea-formaldehyde, melamine-formaldehyde, phenolformaldehyde and similar resins is well established, [see B. Meyer, UF-Resins, Addison-Wesley, Waltham, Mass., 1979, in press] and a wealth of modifications and combinations have been and are being formulated to fulfill specific needs arising from the materials to be bonded, or requirements of product performance. For example, a resin containing approximately 40% melamine, 50% urea and 10% phenol has well known properties which can be correlated to well known application and production standards of earlier or individual components resins [see, J. Mayer and C. Schmidt-Hellerau, German Off. No. 2,020,481, now: U.S. Pat. No. 3,734,918, "Phenol-urea-melamine resin adhesive for wood."].
However, for intrinsic chemical reasons, almost all good and economic resins contain a substantial molar excess of formaldehyde. Some of this is released during the curing of the resin, for example in the hot press during the manufacture of particle board, causing obnoxious fumes. Some is slowly released from the finished product, and imparts upon the product an odor which in many modern applications, e.g. the construction or furniture industry, is considered objectionable. In addition, the resins contain some methylol and other intermediates and reaction products which can readily and reversibly hydrolyse, yielding methylene glycol and, finally formaldehyde. Furthermore, some resins, especially those containing urea-formaldehyde, tend to slowly decompose during aging, hydrolysis, weathering and thereby likewise release objectionable formaldehyde. This formaldehyde release rate is in direct conflict with conditions for optimizing almost all other factors and properties. The release is especially noticeable in particle board and in insulation foams, both of which contain cured resin films with a very large surface and which enhance this release. These films are of uneven thickness, which makes uniform curing very difficult. In particle board, the wood can act like a sponge, holding water in contact with the resin, and the swelling and shrinking during changes in humidity exacerbates the problem.
Several paths have been explored for many years for reducing formaldehyde release, but all entail significant mechanical, chemical or economic disadvantages. For example, a large number of urea-formaldehyde resins have been formulated incorporating sulfite wastes or lignosulfonates [see, for example, E. Roffael & W. Rand, Holzforschung, volume 27, page 178, 1973; and U.S. Pat. No. 3,994,850]. However, in order to obtain products equivalent to traditional urea-formaldehyde resins, an excess of about 1-5% of resin must be applied, a higher curing temperature is necessary and longer curing or press times are required, followed sometimes by autoclaving. Unfortunately, the products tend to be more brittle. Similar problems are encountered if traditional resins are post-cured with urea or phenol in order to reduce the free formaldehyde in the resin [see for example: C. T. O'Neill, U.S. Pat. No. 3,996,190], as all these methods modify the chemistry of the resin structure or skeleton, modify the properties during application, and modify the products. A summary of the problems are contained in H. J. Deppe and K. Ernst, "Taschenbuch der Spanplattentechnik", DRW-Verlag, Stuttgart, 1977. S. Imura and N. Minemura, Hokkaido Fosert Prod. Res. Institute, Rinsan Shikenjo Geppo Vol. 305, pages 1-5, 1977, have reported reduction of formaldehyde odor from finished plywood by washing it with aqueous solutions of several oxyacids of sulfur. Their procedure differs from this invention in that the finished product is treated. This includes wood as well as cured UF-resin. Their procedure is less effective, requires far more effort and might cause corrosion and other problems which are not found in our invention.
Thus, there is a need in the art to provide an additive suitable for simple blending with a wide range of formaldehyde containing resin formulations, such as those currently used or tested for use in making insulating foams or thermo-pressed bonded products. Such an additive should not significantly or adversely affect the properties and behavior of the stored or ready-to-use resin, so that it can be used in every way in an equal manner to that of the resin without addition of the additive. Furthermore, the curing of the bulk, or at least a large fraction of the resin should not be significantly altered, and the chemical structure and skeleton of the ready-to-use resin should not be basically changed, so that a bonded product of equal or better qualities to those of the unmodified resin is obtained, but excess formaldehyde and release of free formaldehyde is greatly reduced, and thus the objectionable odor significantly abated.
Accordingly, an object of the invention is to modify a variety of prior art formaldehyde resins with a sulfur containing additive so as to retain and provide optimum properties as characteristic for the resins, while exhibiting reduced odor during curing in a hot press and/or in the cured resin.
Other objects will also be hereinafter apparent from the description of the invention which follows.
Broadly stated, the above and other objects are realized by adding a sulfur containing compound to ready-to-use, conventional, formaldehyde containing resins, such as for example, urea-formaldehyde, phenol-formaldehyde, melamine formaldehyde, or any cocondensation products such as melamine-urea-formaldehyde, or melamine-urea-phenol-formaldehyde. The sulfur is preferably present in any valance state other than +6, or any intermediate state or combination of such states, e.g., +5 to -2. Sulfur in the oxidation state of six, e.g. sulfate, is not found effective. The sulfur compounds can be introduced alone, in combination, or in combination with other resin components or odor reducing agents such as ammonia, ammonium chloride, quanidine, etc.